Magnetic disk drives provide inexpensive, non-volatile storage of large amounts of electronic data, high rates of data transfer, and high reliability over large numbers of read and write cycles. For these reasons, magnetic disk drives are the predominant mass storage devices in current computer systems. As with any physical material, the surfaces of rotating magnetic media are subject to manufacturing defects and defects that arise during use due to mechanically and electrically induced stresses. In order to enhance the reliability of magnetic media, sophisticated defect-circumventing mechanisms have been developed to map defective data storage regions of a magnetic medium to available, unused, spare data storage regions provided on the magnetic medium. A variety of methods for remapping defective areas have been developed and are currently in use. Most depend on provision of extensive lookup tables that are interspersed with data-containing regions of the magnetic medium.
With the continuing decrease in cost, and increase in capacity, of integrated-circuit electronic memory devices, solid-state, storage devices have become an increasingly fast data storage and data retrieval characteristics of electronic memory are needed. In such applications, the higher data transfer rates of solid-state storage devices with respect to magnetic disk drives may offset and justify the higher cost, per data unit stored, of solid-state storage devices versus magnetic disk drives.
Just as regions of the surfaces of magnet disk drives may contain manufacturing defects, or may become defective through use, data-storage cells within an electronic memory may be defective upon manufacture or may fail during use. Just as in magnetic disk drives, solid-state storage devices need to provide enhanced overall reliability by detecting defective memory cells and providing spare memory cells as substitutes for defective memory cells. However, magnetic data storage medium is relatively cheap, so that use of a relatively large fraction of the physical data storage medium for remapping tables in magnetic disk drives does not significantly increase the overall cost of a magnetic disk drive. Moreover, because of relatively long latency times for data access, arising from the need to mechanically position read/write heads over a target data storage region, complex remapping calculations may be undertaken in magnetic disk drives without significantly increasing access times and decreasing data transfer rates. In solid-state storage devices, by contrast, the physical storage medium is expensive, and therefore the use of a relatively large fraction of the medium for remapping tables can significantly increase the overall price of a solid-state storage device and significantly decrease the solid-state storage device's cost effectiveness in a given application, and complex remapping calculations directly increase access times and decrease data transfer rates. For these reasons, designers, manufacturers, and users of solid-state storage devices have recognized the need for a method and system for dynamically substituting spare memory cells to replace defective memory cells in the solid-state storage device that does not employ large remapping tables and complex remapping calculations.